Marc used to sing Old Macdonald for both kids and it had them quiet. Or he'd do raspberries and play with hands and feet. If you have a cold house it might be worth warming the towel near a heater whilst bubs is in the bath and try and dress them as quickly as possible as its often the shock of the warm water to the cold air that makes them cry more. And if you use a change mat make sure there is no plastic showing as this can be freezing too, so something like a towel covering it will help cause less shock.